Treatment of metal plating wastewaters by electrochemical advanced oxidation process

The electroplating industry is one of the major water polluting industries in the world. Metal complexes, characterized by refractory linkages between heavy metal ions and organic chelating agents, are ubiquitous in the effluents from the electroplating industry. In view of the limited efficacy of these traditional treatment methods, there has been considerable interest in recent years in the use of electrochemical advanced oxidation processes (EAOP) for the treatment of electroplating wastewaters. In EAOPs, highly reactive oxidants such as hydroxyl radicals are produced via water splitting at the anode. The interaction of metal organic complexes with these reactive oxidants and/or direct oxidation at the anode surface oxidizes the organic ligand, forming lower molecular weight compounds with reduced metal binding affinity. As a result, metal ions are released to solution and, if the reducing potential at the cathode is appropriate, is deposited in elemental form at this electrode. Metal-organic complexes can also be directly reduced at the cathode using a sufficiently negative cathodic potential with concomitant release of free ligand into solution.This presentation will highlight some of the key parameters, such as electrode material and geometry, current density and matrix influencing the performance of EAOP process. Furthermore, detailed mechanistic insights into the process, in particular (i) the role of direct electron transfer versus hydroxyl radical-mediated oxidation of metal-organic complexes at the anode (ii) contribution of direct reduction of metal-organic complexes at the cathode to overall metal removal, (iii) various intermediate products formed on degradation of metal-organic complexes, (iv) key reactions limiting the mineralization efficacy of the process and (v) influence of nature and concentration of binding ligands and metal ions on the degradation mechanism, will be provided. Some insights into selective removal of metals from wastewaters containing mixture of metals and approaches for effective recovery of metal ions deposited onto the cathode will also be provided.